This invention relates to distribution networks and in particular to a method and arrangement for facilitating communication in such networks.
Data networks based on packet protocols, such as Internet protocol (IP), use addressing information in the packet header to determine the destination of individual packets. Since each packet is treated independently of any other packet, the networks are connectionless. Packets pass individually through the network between router nodes which determine to which router the packet should be sent next. The processing power required to interpret packet header information and determine the route to the next network node limits the capacity of the router.
A description of an arrangement and method for transporting connectionless traffic is given by Grovenstein et al. in IEEE Network, the Magazine of Computer Communications, vol. 8, No. 6, Nov. 1 1994, pages 18 to 22.
In many instances, a communication between two terminal nodes in a communications network involves transmitting many IP packets in sequence. This is commonly referred to as a xe2x80x98flowxe2x80x99. When a flow is identified by a router, a temporary link xe2x80x98connectionxe2x80x99 can be set up through the network to carry this flow. All packets with the same header information can then pass over this connection avoiding the need to process the headers on an individual basis. This technique is known as IP switching.
Distribution networks for providing multimedia services to subscriber terminals are currently being introduced. One proposed form of distribution network employs ATM (asynchronous transfer mode) as a transport mechanism, and such a network is described in our co-pending United Kingdom patent applications Nos 9602808.9 9602807.1 and 9602809.7. While ATM is suitable as a transport in the core network, the low cost integration of services is at present enabled in the access and customer premises networks by the use of Internet technology for the integration of voice and data. However, the Internet is not adapted to real time communications because of its connectionless paradigm, multiple routing hops and minimum packet length. This can introduce end to end delays thus impairing the quality of service.
It is an object of the invention to provide an improved method of delivering services in a distribution network.
It is a further object of the invention to provide Internet real time communications services.
According to the invention, there is provided a method of transporting connectionless packet traffic together with connection oriented traffic over an asynchronous broad band (ATM) network comprising an access network incorporating a plurality of access units (11) each coupled to a switch or core (12), the method including, providing permanent virtual channels between said access units, multiplexing traffic from a plurality of users into a single minicell connection wherein each said user""s traffic is carried in minicells each provided with a header identifying that user""s channel, establishing a communication path for said minicell connection over a said permanent virtual channel via a adaptation negotiating procedure (ANP), and switching the multiplexed traffic at an ATM adaptation layer associated with the ATM network.
We have found that the eveloving definition of a composite user ATM Adaptation Layer (AAL-2) in ITU SG13, may be used to provide an effective solution to the integration of real-time communications and Internet service. We address the issues of end-to-end connection control, and integration of Internet, telephony and legacy services according to the relevant sections of CFP6.
A feature of our approach, is the ability to provide real-time communications services with a quality equal to that of existing PSTNs over ATM. These services may be multiplexed in the same connections in the access and core networks with Internet protocol packets. The network model is in essence both a switching network for real-time communications, and a router for Internet packets, handling both types of service in a single integrated connection oriented manner. Additionally this obviates the need to perform Internet protocol routing in the ATM network.
ATM is used herein purely as a transport technology with the use of permanent virtual connections (PVC) in the core and access networks. The services are switched in the adaptation layer (AAL-2), rather than in the ATM layer, in a so called adaptation layer switch (ALS), which equates to the LE (local exchange) in the core network in the DAVIC reference model. The ALS responds to standard adaptation negotiation procedures (ANP) of I.363.2 to control connections, and requires no other signalling interface. A Network Adapter, which corresponds to the Access Node in the DAVIC reference model, provides end-to-end connectivity across the core network by using the ANP to control AAL-2 connections in the core that are switched by the ALS.
The Network Adapter responds to proxy signalling and may perform a concentration function. The Network Adapters perform adaptation of real-time communications and Internet to and from the AAL-2 format. Alternatively the adaptation may be performed in the STB by back-hauling the AAL-2 format to the Service Consumer System (SCS).
Further, we have found that AAL-2 can be distributed in MPEG-2 frames, allowing it to be multiplexed directly with MPEG-2 based transport in the access network. This extends one uniform technology from the core network through to the subscriber.
In our arrangement, the AAL-2 adaptation layer of ATM provides a new paradigm for communications and packet transfer. The adaptation layer has been optimised to cope with the demands of low-bit-rate communications, representing the increasing move towards greater compression. In particular, the adaptation layer is a multiplex of users in a single ATM connection, where each user""s information is carried in a short packet, with a header identifying the user channel with ancillary control information.
There are several benefits of this approach, the two main ones being
low-delayxe2x80x94ATM cell assembly delay is a problem that becomes more and more pronounced with lower bit-rate, as it represents a significant chunk of network end-to-end delay budget which is planned to avoid unpleasant voice echo and good performance from interactive services. By having a multiplex of short packets in the payload of a cell, so called mini-cells, the length of those mini-cells can be tailored to the bit-rate of the service, and in general not suffer the ATM cell assembly delay.
high link bandwidth utilisationxe2x80x94by multiplexing several users in a connection oriented manner over a single ATM PVC, and taking account of the statistics of the service type and user call model, allows for a concentration function that ensures a high utilisation of link bandwidth with small or no padding overhead. The ATM connection is effectively an asynchronous dynamic trunk group.
In our arrangement, AAL-2 handles all mobile and compressed voice standards with or without Speech Activity Detection down to bits rates as low as 4 kb/s. It is also designed to handle long data packets by segmentation and re-assembly to and from the short mini-cell format, such that mobile packets can be carried along with the voice.
Essentially, AAL-2 is suitable for all legacy telephony and circuit switched wire line traffic, and all wireless services.
AAL-2 minicells may operate equally over a Switched Virtual Circuit (SVC), or in view of the connection""s xe2x80x9ctrunk groupxe2x80x9d behaviour, over a permanent or semi-permanent ATM VCC. To establish user channels through an AAL-2 based VCC, a straightforward handshake protocol called Adaptation Negotiation Procedures (ANP) is defined in the standard. The ANP operates end-to-end over the VCC, and may be tailored to the user service type. Call establishment and maintenance, and control over the minicell length and other service specific parameters is provided. An F7 OAM flow is defined in the standard to monitor link or end-to-end performance through multiple AAL-2 connections with intervening relay nodes. The OAM can be tailored to the service type rather than being a part of the transport layer.
To establish user channels through an AAL-2 PVC, a straightforward handshake protocol called Adaptation Negotiation Procedures (ANP) is defined in the network standard. The ANP operates end-to-end over a VCC, and may be tailored to the user service type. Set-up, tear-down and modification are available operations, with control of the mini-cell length and other service specific parameters. An F7 OAM flow is defined in the standard to monitor end-to-end performance through multiple AAL-2 connections with intervening switching nodes. The OAM can be tailored to the service type, rather than being a part of the transport layer.
In particular, we exploit the generic capability of AAL-2, which is an asynchronous connection-oriented short packet protocol, where ATM provides virtual connectivity in the transport layer. This allows real-time traffic to be easily integrated on the same links with data and to be switched. For example, the real-time traffic may be a free statistical mixture of voice and video at any compression ratio, allowing full multicast video conferencing to be readily implemented.
A further benefit of the use of AAL-2 in the present arrangement and method is that, because of its multiplexed packet capability, mini-cells can be switched between ATM connections. Thus only PVC capability in the ATM core network is necessary, as all switching and concentration of a service can be performed at the adaptation layer. Adaptation layer switching allows a network to be scaled, and to be managed on a service basis. An ATM based network with adaptation layer switching means that ATM is retained as a transport technology, or as a switching technology for services not addressed in this manner.
The technique is applicable to distribution networks such as the network protocol defined by the DAVIC standard, but it is of course in no way limited to that application.